Separate HTML response generation logic out of getBridgeRequestAnswer().

 * MOVE logic for generating HTML responses out of
   bridgedb.HTTPServer.WebResource.getBridgeRequestAnswer() and into a
   new method, bridgedb.HTTPServer.WebResource.renderAnswer(). This
   allows other methods to generate responses, even if the request was
   malformed.
Add exception-catching wrapper to bridgedb.HTTPServer.render().

 * ADD a wrapper which catches all unhandled exceptions which occur
   while generating an HTML response. If such an exception occurs, this
   ensures that the error is logged and the default "No bridges
   currently available" response is served to the client (rather than
   display a traceback to the client, which is what BridgeDB did
   previously).
 * FIXES #6127
Merge branch 'fix/6127-web-server-tracebacks' into develop